Objective  Soil micro-aggregates are the important carriers of phosphorus(P) storage and turnover. To clarify the form and bioavailability of soil microaggregate associated P in upland red soil under long-term fertilization can provide the theoretical basis for the efficient and rational application of P fertilizers.

  Method  Based on the long-term fertilization experiment (1988 - 2014) located at the Red Soil Ecological Station in Yingtan, Jiangxi Province, China, total 15 soil surface samples (0 - 15 cm) were collected from the combined application of organic-inorganic fertilizer plot (Treatment 1/2 NPK (CK), low quantity NPK + rice straw (RS) and low quantity NPK + pig manure (PM)) and the single application of chemical fertilizer plot (Treatment NPK and NK), respectively. Soil micro-aggregate size in 0.25 - 0.05 mm, 0.05 - 0.01 mm, 0.01 - 0.005 mm and < 0.005 mm were sequentially extracted by pipette method after the water disperses. The changes of soil total P (TP), Bray-P and the P activation coefficient (PAC) in various soil micro-aggregates were analyzed, and the extremely available P (EAP), medium available P (MAP) and non-available P (NAP) of micro-aggregates response to the long-term fertilization were also analyzed based on the results of soil P fractionation. And the relationships between the soil P fractionation, PAC, particle composition, soil organic matter, iron-aluminum oxides and Bray-P were discussed based on the structural equation model.

  Result  Long-term combined application of pig manure can significantly increase the TP, Bray-P, PAC and EAP, MAP, NAP in various micro-aggregates, and the increase proportion of them in < 0.005 mm micro-aggregate was higher, but the combined application of rice straw only can significantly decrease the P content of 0.01 - 0.005 mm micro-aggregate. Compared with NPK treatment, NK treatment can significantly decrease the contents of all P fractions in various micro-aggregates, and also can significantly decrease the PAC of 0.25 - 0.05 mm and 0.05 - 0.01 mm micro-aggregates, but significantly increased the PAC of 0.01 - 0.005 mm and < 0.005 mm micro-aggregates. The fitting results of structural equation model show that the inorganic P component was the positive influence factor for Bray-P, but the organic P component was the negative influence factor for Bray-P. Based on the stepwise regression analysis, the inorganic P components extracted by the NaHCO₃ and HCl were the main influence factors for PAC of upland red soil.

  Conclusion  The combined application of organic fertilizers, especially combined with pig manure, can significantly increase and promote the transformation of the high stable organic P components into low stable inorganic P in upland red soil.